Method for detecting the quantization of spectra

ABSTRACT

The invention comprises the retrieval of a quantized spectrum of audio-data, which had been compressed conforming to psychoacoustic principles. This spectrum can be identified with the aid of sorted (and logarithmized) differences of the magnitude of the amplitude. A problem to be solved by the invention is to find the basis of sensitive indicators for the degree of quantization of a spectrum. The result of this calculation shows for a quantized spectrum several small values at amplitudes of the same quantization level, and obvious spikes at positions, where the amplitudes have reached a higher level of quantization, while a non-quantized spectrum typically resembles noise.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/EP02/13188, filed Nov. 23, 2002, which waspublished in accordance with PCT Article 21(2) on Jun. 12, 2003 inEnglish and which claims the benefit of European patent application No.01250431.2, filed Dec. 6, 2001.

BACKGROUND

Audio-data are being transmitted in a compressed way conforming topsychoacoustic principles and decoded by the receiver. Re-compressing ofthese decoded data conforming to psychoacoustic principles will lead toloss of quality. This loss can be omitted, if the parameters of theoriginal compression can be detected and re-used, see e.g. EP-A-0746913.

A method for the detection of these parameters for codecs which usespectra, is testwise calculation of spectra with different parameters.If the parameters of analysis (start of transformation, window type,length of spectrum) match the codec parameters, the spectrum has aquantized appearance. Because of calculation impreciseness or differentcalculation precision of the codec and of the analysis calculation, thequantized spectrum will look slightly noisy, i.e. the amplitudes of thesame level of quantization will NOT have the same magnitude.

The difficulty is to distinguish, whether a spectrum has a “quantizedcharacter”, so that different parameters of the spectrum can be analyzedconcerning their impact on the quantization of the spectrum.

Because compression goes along with a vanishing of certain lines of thespectrum (cf. in Herre, Jürgen; Michael Schug: Analysis of DecompressedAudio—The “Inverse Decoder”, 109th AES Convention, Los Angeles 2000)those spectral lines are being counted, which are smaller than athreshold value.

INVENTION

The invention comprises the retrieval of a quantized spectrum ofaudio-data, which had been compressed conforming to psychoacousticprinciples. This spectrum can be identified with the aid of sorted (andlogarithmized) differences of the magnitude of the amplitude.

A problem to be solved by the invention is to find the basis ofsensitive indicators for the degree of quantization of a spectrum. Theresult for a quantized spectrum shows several small values at amplitudesof the same quantization level, and obvious spikes at positions, wherethe amplitudes have reached a higher level of quantization, while anon-quantized spectrum typically resembles noise.

This problem is solved by the method disclosed in claim 1.

In principle, the inventive method is suited for tandem codecs(decoding-encoding). The invention can also be used at the procedure ofreverse engineering with unknown codecs.

Advantageous additional embodiments of the invention are disclosed inthe respective dependent claims.

DRAWINGS

Exemplary embodiments of the invention are described with reference tothe accompanying drawings, which show in:

FIG. 1 Magnitudes of a quantized spectrum of a decoded audio signal;

FIG. 2 Magnitudes of a non-quantized spectrum;

FIG. 3 Amplitudes of FIG. 1) sorted;

FIG. 4 Amplitudes of FIG. 2) sorted;

FIG. 5 Differentiated sorted spectrum from FIG. 3);

FIG. 6 Differentiated sorted spectrum from FIG. 4);

FIG. 7 Simple block diagram of the detector.

EXEMPLARY EMBODIMENTS

FIG. 1 shows the magnitudes of a quantized spectrum of a decoded signal.The parameters of analysis (start of the transformation, window type,length of spectrum) match the codec parameters, therefore the spectrumhas a quantized appearance.

In comparison to FIG. 1, FIG. 2 shows the magnitudes of a non-quantizedspectrum. Thus, the parameters do not match the codec parameters.

FIG. 3 shows the sorted amplitudes from FIG. 1, while FIG. 4 shows thesorted amplitudes of a non-quantized spectrum from FIG. 2.

FIG. 5 shows the differentiated sorted spectrum from FIG. 3, while FIG.6 shows the differentiated sorted spectrum of a non-quantized spectrumfrom FIG. 4

The comparison of FIGS. 5 and 6 will clearly show the difference betweena quantized spectrum (FIG. 5) and a non-quantized spectrum (FIG. 6).

The next step is to calculate a scalar out of these spectra, which willbe used as an indicator for the quantization.

The invention can be used in connection with an inverse encoder. Oftenan encoded and decoded audio signal has to be encoded for anothertransmission a second time. If the encoder parameters in the secondencoder are the same as in the first, codec artifacts can be minimized.The inverse encoder tries to retrieve these parameters. In this case,for it has to be decided, if a spectrum looks quantized, which can bedone with the help of the invention.

Another use of the inverse encoder and consequently also of theinvention is in reengineering codecs for proving infringements in thearea of intellectual properties.

FIG. 7 shows a simple block diagram of the invention. The purpose of theinvention is to convert time domain samples in frequency domain viaspectral converter 1, then sorting the amplitudes in frequency domainvia sorting device 2, followed by an optional logarithmizing of thesesorted amplitudes, via logarithmizer 3, and finally assessment of sortedspectra values via assessment element 4, e.g. differentiation anddetection of first peak.

1. Method of detecting the quantisation of spectra of a decoded audio signal in order to detect or retrieve encoding parameters that were used in the encoding of said audio signal, said method including the steps: converting samples of the decoded audio signal into frequency domain, thereby using the test parameters start position of transformation, window type, length of spectrum; in the frequency domain, sorting the magnitudes of the spectral values; differentiating the sorted spectral magnitude values; assessing for a given set of test parameters whether the sorted spectral values have a quantised appearance, using the position and/or height of a first peak in said differentiation.
 2. Method according to claim 1, wherein the spectral values are logarithmised (3) before said assessing step.
 3. Apparatus for detecting the quantisation of spectra of a decoded audio signal in order to detect or retrieve encoding parameters that were used in the encoding of said audio signal, said apparatus including: means for converting samples of the decoded audio signal into frequency domain, thereby using the test parameters start position of transformation, window type, length of spectrum; means for sorting, in the frequency domain, the magnitudes of the spectral values and for a differentiating the sorted spectral magnitude values; means for assessing for a given set of test parameters whether the sorted spectral values have a quantised appearance thereby using the position and/or height of a first peak in said differentiation. 